The processing of semiconductor wafers involves processing tools for performing a large number of processing steps. The processing steps are usually performed in a vacuum chamber. The processing tools typically handle and process wafers one at a time in order to optimize control and reproducibility. These processing tools utilize automated wafer handling.
The throughput of processing tools is an important factor in achieving low cost manufacture. Ion implantation processing is one such processing step in which the efficiency of the wafer handling is critical to the overall wafer throughput. Wafer handling involves introduction of the wafers in a cassette or other wafer holder into the processing tool. The processing tool will typically remove the wafers from the cassette or other wafer holder and transport the wafers in the vacuum chamber, typically through a load lock, and then further transfers the wafers to a processing station. Upon completion of processing the wafers, the wafer handler will transport the wafers back to the cassette or wafer holders. Some of the processing and wafer handling operations may be performed concurrently to achieve efficient operation and high throughput. Careful design of wafer handling systems is required and a variety of wafer handling techniques are known in the prior art.
In one prior art system disclosed by Tamai in U.S. Pat. No. 5,929,456, a first group of wafers and a second group of wafers are rotated along first and second orbital paths intersecting a path of an ion beam during ion implantation processing. A wafer from each of the two groups is transferred to first and second wafer holders that respectively move the wafers on orbital paths CL1 and CL2 with at least a portion of the second orbital path being different from the first orbital path as shown in FIGS. 1A–1D of the Tamai patent. After one of the wafers completely traverses an ion beam radiation region 4, the wafer is transported upward as indicated by the broken lines in FIGS. 1B and 1D, while the other wafer traverses the radiation region. These processes are repeated until ion implantation of the two wafers is completed. Once the two wafers are implanted, two new wafers are transferred from load locks to replace the two implanted wafers on the wafer holders 50A and 50B. The two wafers are continuously rotated in front of the ion beam until the ion implantation processing is completed. Once the ion implantation processing is completed, the next set of unprocessed wafers replaces the processed wafers. These cycles are repeated until all of the wafers from the load lock are processed. However, this system suffers from relatively low wafer throughput because the ion beam is not fully utilized. During transfer of the sets of wafers, the beam is not being utilized for implanting into the wafer. Accordingly, there is a need for improved wafer handling systems which maximize the utilization and the time that ion beam is implanting onto the wafers or workpieces.